Method of liquid reversal development for electrography

ABSTRACT

A BIAS VOLTAGE HAVING AN OPPOSITE POLARITY RELATIVE TO TONER IS FIRST APPLIED TO A DEVELOPER LIQUID TO THEREBY CAUSE THE TONER TO MOVE TOWARD A DEVELOPMENT ELECTRODE, AND A BIAS VOLTAGE HAVING THE SAME POLARITY AS THE TONER IS THEN APPLIED TO THE DEVELOPER LIQUID. THIS METHOD INSURES IMPROVED REPRODUCTIVITY OF CONTRAST AND HALFTONE OF IMAGES ON ELECTROGRAHIC PAPER.

Nov. 21, 1972 susuMu TANAKA E.TAL 3,703,399

ME'IHQD OF'LIQUID REVERSAL DEVELOPMENT FOR ELECTROGRAPHY Filed Oct. 14.1970 Inwnlor JKSWW TYIKHKH Kl-- Km wnun Allorne y United States Patent3,703,399 METHOD OF LIQUID REVERSAL DEVELOPMENT FOR ELECTROGRAPHY SusumuTanaka and Ken-Ichi Wada, Sakai, Japan, assignors to Minolta CameraKabushiki Kaisha, Minamiku, Japan Filed Oct. 14, 1970, Ser. No. 80,614Claims priority, application Japan, Oct. 29, 1969, 44/ 86,577 Int. Cl.603g 13/10, 15/10 US. Cl. 11737 LE Claims ABSTRACT OF THE DISCLOSURE Abias voltage having an opposite polarity relative to toner is firstapplied to a developer liquid to thereby cause the toner to move towarda development electrode, and a bias voltage having the same polarity asthe toner is then applied to the developer liquid. This method insuresimproved reproductivity of contrast and halftone of images onelectrographic paper.

BACKGROUND OF THE INVENTION The present invention relates to a method ofliquid reversal development for electrography.

Liquid reversal development in which toner is deposited on the unchargedportion of an electrographic paper unlike in normal development employsa method wherein a bias voltage E having the same polarity as toner 4dis persed in an insulating liquid 3 is applied between a base electrode2 and a development electrode 1 so as to force the toner 4 to deposit onthe uncharged portion A of an electrographic paper 5.

In accordance with such bias method, however, the charged portion B ofthe electrographic paper 5 is not always charged perfectly uniformly,but due to uneven size of fine particles of semi-photoconductor on theelectrographic paper or uneven distribution of fine particles ofsemi-photoconductor on resin-dispersed electrographic copying paper,there arises a delicate uneven charge density, giving rise to localelectric field disorder in many cases. As a result, the bias voltage B,when increased to the same level as the surface potential of the chargedportion B, produces deposition of toner 4 also on the charged portion Band exerts an adverse effect on the image obtained.

FIG. 1 schematically illustrates such situation. The charged portion Bof electrographic paper 5 is negatively charged, and toner 4 in thedeveloper liquid is of a negative polarity. When a negative voltagewhich is equal to the surface potential of the charged portion B isapplied as the bias voltage E, the toner 4 which is dispersed uniformlyin the insulating liquid 3, in the area of uncharged portion A, is movedtoward and deposited on electrographic paper 5 under the influence ofthe bias voltage E as shown in FIG. 1b. On the charged portion B, a weakelectric field is produced where the surface potential of the chargedportion B is lower than the bias volt-age E due to delicate unevencharge density, the electric field causing the toner 4 to move towardthe electrographic paper 5. Consequently, the toner 4 positioned underthe chain line I shown FIG. 1a, for instance, is deposited on theelectrographic paper 5 as seen in FIG. lb.

Whereas it is possible, in the foregoing bias method, to efifectdeposition of the toner 4 on the uncharged portion A to an extent ashigh as is achieved by normal development, the deposition of toner 4,formation of so-called foggy background, is inevitable in the chargedportion B due to the presence of delicate uneven charge density. Forthis reason, it is usually necessary to consider such "ice uneven chargedensity and to set the bias voltage at a negative voltage which isdetermined paying due consideration to the lowest value of the surfacepotential of the charged portion B.

Thus, liquid reversal development is generally inferior to normaldevelopment in the reproductivity of contrast and halftone of images.

SUMMARY OF THE INVENTION An object of the present invention is toprovide a method of liquid reversal development for electrographs whichinsures reproductivity of contrast and halftone of images equal orsuperior to that achieved by normal development, more particularly toprovide a method of liquid reversal development which does not producedeposition of toner on the charged portion of electrographic paper dueto delicate uneven charge density even when the bias voltage E in theforegoing bias method is set at a level equal to the surface potentialof the charged portion of electrographic paper.

The bias method for liquid reversal development of this invention ischaracterized in that a bias voltage of an opposite polarity relative totoner is first applied to a developer liquid to thereby cause the tonerto move toward a development electrode, a bias voltage of the samepolarity as the toner thereafter being applied to the developer liquidfor removal development of an electrographic paper to obtain a desiredelectrograph. Thus, by bringing the toner in the developer liquid to thedevelopment electrode at the first stage as described above, reversaldevelopment can be efiected under a bias voltage which is equal to thesurface potential of the charged portion even where there existsdelicate uneven charge density on the charged portion of electrographicpaper, thereby making it possible to obtain satisfactory reproductivityof contrast and halftone of images which is equal or superior to thatobtained by normal liquid development. In accordance with thisinvention, it is further possible to obtain high contrast images easilyby overcharging electrographic paper. Thus the present invention hasthese and various other advantages which can not be obtained withconventional method of liquid reversal development.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a view illustrating asituation in accordance with a conventional bias method for liquidreversal development in which fogging takes place; and

FIG. 2 is a view illustrating the principle of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT With reference of FIG. 2, thepresent invention will be described below. FIG. 2a shows a step of thisinvention for moving toner 4 in a developer liquid away fromelectrographic paper 5. This procedure is eifected by applying a biasvoltage E of a polarity opposite to that of toner 4 between adevelopment electrode 1 and a base electrode 2. In the figure, toner 4is of a negative polarity, while the bias voltage E is ofa positivepolarity. Due to the application of the bias voltage E the toner 4 isbrought close to the development electrode 1, with the result that thedeveloper liquid is divided into a layer of toner 4 and a layer ofinsulating liquid 3 as shown in FIG. 2b. Since no toner 4 deposits onthe electrographic paper 5 during this step, the above procedure may beconducted Without placing the electrographic paper on the base electrode2. Further because the bias voltage E serves merely to move the toner 4in the developer liquid toward the development electrode 1, the voltagemay suitably be selected, but preferably, it may usually be set at thesame value as the bias voltage E to be applied in the subsequent step ofdevelopment.

FIG. 2b shows a procedure of reversal development in accordance with theaforementioned bias method which is conducted after the toner 4 has beenconcentrically brought to the development electrode 1 in the above step.When the bias voltage E equal to the surface potential of theelectrographic paper 5 and having the same polarity as the toner 4 isapplied, the toner 4, under the influence of the bias voltage E, ismoved toward and deposited on the electrographic paper 5 at theuncharged portion A as shown in FIG. 2c, while at charged portion B, aweak electric field produced by delicate uneven charge density acts tomove the toner toward the electrographic paper. However, since no toner4 is dispersed in the area under the chain line I in FIG. 2b, nodeposition of the toner takes place on the charged portion B as seen inFIG. 26. The above developing step insures uniform deposition of toner 4on the uncharged portion'A, which results in good reproductivity ofcontrast and halftone of images.

The method of this invention will now be described with reference to anexample as it was actually practiced.

Electrofax paper was negatively charged in the dark by corona dischargeuntil blue white light spot was observed on the charged surface, and anexposure was made, whereby an electrostatic image having surfacepotentials of -400 v. at dark portion and 0 v. at bright portion wasformed. The electrofax paper was then placed on a base plate, and a biasvoltage of +400 v. was first applied to a development electrode for 0.1to 1 second. A bias voltage of 400 v. was thereafter applied to thedevelopment electrode for 0.5 to 2 seconds. When the electrofax paperwas finally taken out, a visual image having high contrast and free offogging was obtained.

In the case where a negative bias voltage alone is applied to thedevelopment electrode under the same conditions as above but withoutconducting the first step, fogging with numerous black spots wereproduced in the dark portions of the image on the electrofax paper takenout.

Further in accordance with the reversal development of this invention,no fogging was seen to take place in the background even whereelectrofax paper was overcharged at 600 v.

We claim:

1. The method of developing an electrostatic latent image of a firstpolarity on a substrate comprising establishing, in a liquid havingdispersed therein a toner of said first polarity, a first electric fieldin a first direction to effect the flow of said toner from apredetermined zone along and adjacent the face of said substrate bearingsaid latent image to deplete said zone of said toner, and thereafterestablishing a second electric field in said liquid opposite to saidfirst direction to effect the flow of said toner toward said substratepositioned in said zone and to deposit said toner on the uncharged areasof said substrate.

2. The method of claim 1 wherein said electric fields are established bypositioning a pair of first and second electrodes in said liquid andapplying corresponding voltages between said electrodes, said firstfield being effected by applying a potential to said first electroderelative to said second electrode of a polarity opposite to that of saidtoner to establish said depleted zone proximate said second electrodeand said second field being effected by applying a potential to saidfirst electrode relative to said second electrode of the same polarityas said toner.

3. The method of claim 2 wherein said voltages producing said first andsecond fields are substantially equal and opposite and of substantiallythe same value as the potential level of said electrostatic image.

4. The method of claim 1 wherein said substrate is positioned in saidzone following the depletion thereof of said toner.

5. The method of claim 1 wherein said substrate is positioned in saidzone prior to the depletion thereof of said toner.

References Cited UNITED STATES PATENTS 3,560,203 2/1971 Honjo et al. 961Y 3,679,453 7/ 1972 Katagiri et a1 l1737 LE OTHER REFERENCES 270,4875/1963 Australia 117-37 LE 2,004,683 11/ 1969 France 117-37 LE WILLIAMD. MARTIN, Primary Examiner M. SOFOCLEOUS, Assistant Examiner U.S. Cl.X.R.

118-637, Dig. 23; 355-10

